The present invention relates generally to parachutes and more particularly to parachutes which include a reefing mechanism for use in packing and opening the parachute canopy in a controlled manner.
Parachutes are well-known devices used by, among others, the military to safely airdrop cargo and/or personnel from what would otherwise be excessively high altitudes. One type of parachute comprises a generally circularly shaped canopy, the canopy having an outer periphery referred to as a "skirt" or a "mouth" and optionally having an apical opening referred to as a "vent." In addition, the canopy typically includes a plurality of regularly spaced seams referred to as "radials" which extend radially from the vent to the skirt and which partition the canopy into sections referred to as "gores." Suspension lines are attached at one end to the skirt at each of the radials and are bound together at the other end at a confluence point which is adapted for attachment to the payload.
As can readily be appreciated, parachute canopies must be capable of being packed in a condition ready for deployment when not in use and of being opened in a controlled yet rapid manner when in flight and exposed to the surrounding air flow field. To meet both of these objectives, various structural adaptations to parachutes have been devised which cause the parachute canopy to respond to exposure to the surrounding flow field in different ways. Such adaptations have included techniques commonly referred to as skirt-reefing, pull-down center line, crown chute, and secondary parachute at skirt.
In "RAPID--The Design of a Low Altitude Parachute," by Elsa J. Hennings, presented at the 11th AIAA Aerodynamic Decelerator Systems Technology Conference, San Diego, CA (Apr. 9-11, 1991), a unique deployment/reefing system for a parachute is described. The system involves packing the parachute with the skirt pulled up to a concentric band located one-third of the way down the gore from the vent. This is accomplished by attaching internal lines that run from each skirt-radial joint through rings at each concentric band-radial joint and in to a single reefing webbing. When this reefing webbing is pulled completely down so that the skirt is drawn up to the concentric band, the drag area of the parachute is very small. The smaller the drag area, the faster the parachute inflates; therefore the parachute opens very quickly when deployed in this configuration. The reefing webbing is designed to be attached to a device that will let it pay out, allowing the parachute to inflate until a preset maximum drag force is reached. When that force is reached, the webbing is locked in place, which stops the inflation process. When the velocity decays, the drag force will drop below the preset maximum force, so more webbing will be let out, allowing additional inflation. This sequence continues until the parachute is fully inflated. In the low speed case, the maximum force would not be reached, so the reefing webbing would pay out completely upon parachute deployment.
The skirt, which initially opens to the diameter of the concentric band (not completely shut as in normal deployment), drops straight down and is inflated very quickly. At low speeds, this system allows for a very fast opening, while at high speeds, the reefing webbing controls the inflation, keeping the opening force from exceeding human tolerance. Unlike the pull-down vent line technique, this deployment method causes the opening loads to be highest at the vent, which is designed to withstand such forces. In addition to the controllable reefing webbing, slots at each radial seam running from the skirt to the concentric band act to automatically reduce excessive forces by opening when the pressure inside the parachute is high (during the inflation process) and closing when the pressure drops (during steady state descent). This results in the maximum drag area (slots closed) when it is needed most, i.e., when the parachute is close to the ground.